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In microeconomic theory, the opportunity cost of a choice is the value of the best alternative forgone, in a situation in which a choice needs to be made between several mutually exclusive alternatives given limited resources. Assuming the best choice is made, it is the "cost" incurred by not enjoying the benefit that would be had by taking the second best choice available. The New Oxford American Dictionary defines it as "the loss of potential gain from other alternatives when one alternative is chosen". Opportunity cost is a key concept in economics, and has been described as expressing "the basic relationship between scarcity and choice". The notion of opportunity cost plays a crucial part in ensuring that scarce resources are used efficiently. Thus, opportunity costs are not restricted to monetary or financial costs: the real cost of output forgone, lost time, pleasure or any other benefit that provides utility should also be considered opportunity costs.
The term was coined in 1914 by Austrian economist Friedrich von Wieser in his book Theorie der gesellschaftlichen Wirtschaft. It was first described in 1848 by French classical economist Frédéric Bastiat in his essay "What Is Seen and What Is Not Seen".
Explicit costs are opportunity costs that involve direct monetary payment by producers. The opportunity cost of the factors of production not already owned by a producer is the price that the producer has to pay for them. For instance, a firm spends $100 on electrical power consumed, their opportunity cost is $100. 
Implicit costs (also called implied, imputed or notional costs) are the opportunity costs implied by the uses to which the actor (i.e., firm) allocates their existing (owned) resources, or factors of production. For example: a manufacturer has previously purchased 1000 tons of steel and the machinery to produce a widget. The opportunity costs of selling the steel and renting the machinery (instead of producing the widget) are explicit costs.
Note that opportunity cost is not the sum of the available alternatives when those alternatives are, in turn, mutually exclusive to each other – it is the value (that is, the benefit) of the next best alternative. The opportunity cost of a city's decision to build the hospital on its vacant land is the loss of the land for a sporting center, or the inability to use the land for a parking lot, or the money which could have been made from selling the land. Use for any one of those purposes would preclude the possibility to implement any of the other.
Q: Suppose you have a free ticket to a concert by Band A. The ticket has no resale value. On the night of the concert your next-best alternative entertainment is a performance by Band B for which the tickets cost $40. You like Band B and would usually be willing to pay $50 for a ticket to see them. What is the opportunity cost of using your free ticket and seeing Band A?
A: The benefit you forgo (that is, the value to you) is the benefit of seeing Band B. As well as the gross benefit of $50 for seeing Band B, you also forgo the actual $40 of cost, so the net benefit you forgo is $10. So, the opportunity cost of seeing Band A is $10.